I have come up with a graphic design for shirts that I am interested in marketing and protecting, but have some questions.

The design on the front is entirely my own. The back is what I am unsure about. I modified a digital version of a National Park map. From what I could find, these maps are in the public domain. The site below says that unless otherwise stated, the map is in the public domain. I cannot find any statement of that sort. http://www.nps.gov/disclaimer.htm The map is the [1.8 MB PDF] detailed park map found here: http://www.nps.gov/yose/planyourvisit/maps.htm If it turns out to be protected, then do my modifications make it my own creation? Thanks for your help!